Bethesda has obtained the rights to develop Fallout, they've licensed the Fallout franchise from Interplay (except the MMO license)

Well let's get started shall we, first let's do some history lessons:

IPLY originally begun working on Fallout 3 just after Fallout 2, but it ended up being scrapped.

IPLY again started working on Fallout 3 in 2003 and called the project Van Buren, which also ended up being scrapped.

Bethesda purchased the right to produce Fallout 3 in July 2004, with an option to purchase Fallout 4 and 5. They do not own the entire license, Interplay still does, and Interplay holds the right to online Fallout games (FOOL), Bethesda has now begun pre-production on Fallout 3. "Full production" started in late 2006.

As of the 8th of April 2007, Bethesda owns the full Fallout license, witrh Interplay only retaining a limited right to produce a Fallout MMO.